Add ability to disable default form wrapper

[spohlenz]

This PR adds the ability to disable the default form wrapper (container <div> including sidebar), and instead leave it to the form partial to lay out the container. This can be useful for laying out a form in sections, or within a grid.

Trestle.resource(:articles) do
  form wrapper: false
end

[coveralls]

coverage: 91.421% (+0.005%) from 91.416%
when pulling 1a7d379 on disable-form-wrapper-option
into dc8e987 on main.